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    <title>Document</title>
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    <script>
        // 将数组 ['关羽', '张飞', '程咬金', '吕布']; 的内容反过来存放
        //       ['吕布', '程咬金', '张飞', '关羽']
        let arr = ['关羽', '张飞', '程咬金', '吕布'];
        let arr1 = [];
        let arr2 = [];
        /* 
            index0   index1
               0        3
               1        2
               2        1
               3        0
              index0 + index1 = 3(arr.length -1)
              index1 = arr.length - 1 - index0
        */
        for (let index in arr) {
            // console.log(index, arr[index]);
            arr1[arr.length - 1 - index] = arr[index];
        }
        console.log(arr1);

        console.log('-------------方案2---------------');

        // 倒序遍历数组，再将元素放入新数组。
        
        for (let i = arr.length - 1; i >= 0; i--) {
            // 倒序遍历
            // console.log(i, arr[i]);
            arr2[arr2.length] = arr[i];
        }
        console.log(arr2);
    </script>
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